2.5" into 3" dp. . .What's the point??

[weaver]

I just bought a megan 2.5 - 3 inch dp and got to thinking if there would really be any beinifit to run 3" over 2.5" piping for the rest of the exhaust since it starts out with a 2.5 inch restriction anyways? I was going to get 3" piping, but if it won't make any difference from the 2.5" then i'll just go with that. What does everybody think??

Thanks!

 

[JDNSBR]

What turbo are you running? If you are running a 16g or something similar like that and want to stick with that, then yes 2.5 will work fine for you. If you are looking to go bigger then I would suggest going to 3". The less restriction you have the faster you can move the exhaust gas. This then will allow for a little quicker spool on the turbo since there is no longer that restriction. This will also allow higher HP numbers because you are no longer choking your engine by restricting the hot exhaust gas.

 

[weaver]

I've got a E316G. So you think 2.5 would be the best choice? I was just wondering if the 2.5" restriction to the flex on the downpipe would negate the benifit of running 3" from the downpipe back?

 

[xveganxcowboyx]

A short restriction is better than an 8 foot long one. The downpipe flange and O2 housing outlet are only 2.5" and people still see gains with a bigger exhaust. Go with the 3" and you can always swap down pipes later and even port the O2 to match.

 

[weaver]

Yeah, I guess that would be the best thing to do. I can't find any mandrel benders in my little town. What bends would I need to buy to complete a cat-back on a 2g GST?

 

[xveganxcowboyx]

I ordered online. There are lots of sellers including Road Race Engineering, JC Whitney, and several ebay sellers (I went with an ebay seller). I'm not sure how many bends you'll need. I only needed 180 for the DP and cat back. I imagine you will need more. The 2G and FWD downpipes are a little more complicated. I also think your enhaust is routed over the rear suspension somehow.

Best thing to do (aside from checking VFAQ to see if there are instructions) would be to look at photos of aftermarket systems and see how many bends they have. Also look at the stock routing and try to see how you might run the new piping.

 

[weaver]

Would there be problems to routing it straight under the axles? I'm not lowering my car or anything like that so would there be enough clearance that way?

 

[housegsx]

From RRE...

DPs have a 2 1/2" stainless steel first bend with a 2 1/2" stainless steel flex section. They then expand to 3" right after the flex section right under the oil pan area. Why start out at 2 1/2"? If you are running on the stock O2 sensor housing, even ported it is 2 1/2" inside diameter. When the exhaust gasses are hot, they flow better, 2 1/2" is plenty for the first 12". As the gasses cool , bigger helps. By going to 3" after the flex section, we allow this expansion and also allow for better clearance at the front where room is tight.

 

[weaver]

Thanks alot, that makes complete sense. Hot air flows faster than cool are so you need less volume pipe to equal the same flow. So by the time the air cools down I need a larger pipe to continue that same flow. Is that about right?

 

[housegsx]

You got it. -

 

[kenamond]

Thanks alot, that makes complete sense. Hot air flows faster than cool are so you need less volume pipe to equal the same flow. So by the time the air cools down I need a larger pipe to continue that same flow. Is that about right?

No. Hot gas is less viscous than cold gas, so it flows easier in the same diameter pipe than cold gas would. The gas actually shrinks as it cools not expands, so cooler gas is moving slower which decreases pressure loss. So colder gas has one benefit of moving slower, but hot gas has the benefit of lower viscosity. I'm not sure which one wins.

 

[white93pgtze]

the more exhaust gases leaving faster makes more hoursepower

 

[kenamond]

the more exhaust gases leaving faster makes more hoursepower

Upwind of the turbine, yes, downwind, no. If you could bleed every last ounce of energy from the exhaust gasses entering the turbine wheel, you'd have 0psig in the exhaust (which is impossible unless you have no exhaust, and is still impossible, because no such turbine wheel exists), but you'd get the most power. You don't want to lose heat in the exhaust mani and turbine inlet, because that is losing energy that can be used to drive the compressor. You want the least restriction after your turbine wheel to maximize pressure differential across the wheel and maximize torque to the compressor.

 

[nightspeed87]

Upwind of the turbine, yes, downwind, no. If you could bleed every last ounce of energy from the exhaust gasses entering the turbine wheel, you'd have 0psig in the exhaust (which is impossible unless you have no exhaust, and is still impossible, because no such turbine wheel exists), but you'd get the most power. You don't want to lose heat in the exhaust mani and turbine inlet, because that is losing energy that can be used to drive the compressor. You want the least restriction after your turbine wheel to maximize pressure differential across the wheel and maximize torque to the compressor.

well put sir


( Besides get 3in.... Might as well if theyre going to cost the same pretty much,
and you never know your hp needs in the future... 3in will def be better... )

 

[weaver]

3" it is!! Thanks for all the replies!!